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Tiếng nói mạng xu hướng và nhu cầu Sự bùng nổ của Internet và sự gia tăng nhanh chóng về số lượng người sử dụng Internet đã không đi không được chú ý. Điều gì tác động sự phát triển này sẽ có trên sự tiến hóa của công trình viễn thông hiện có mạng lưới? Khối lượng của lưu lượng dữ liệu đang tăng lên với tốc độ nhanh hơn nhiều so với giọng nói hoặc lưu lượng điện thoại (Hình 1). Trong những năm gần đây, khối lượng buôn người fic của điện thoại là lớn hơn nhiều so với số liệu, dữ liệu và kết nối mạng dựa ly duy về cách sử dụng các mạng viễn thông dựa trên chuyển mạch, hoặc sử dụng các mạng riêng biệt. Tại một số giai đoạn, có một chéo của giao thông, chẳng hạn như khi cả hai loại giao thông chiếm cùng một lượng năng lực sản xuất trong mạng. Tuy nhiên, giọng nói vẫn chiếm một mức độ lớn của nhà điều hành enue rev. Nhưng lưu lượng thoại hữu tuyến đang tăng chậm hơn ngày hôm nay, và có rất nhiều những lí do tại sao. Một phân khúc phát triển của mạng lưới làm việc bằng giọng nói được xử lý trong các mạng di động. Ngoài ra, khả năng mới tồn tại để di chuyển tất cả mọi thứ đó là dữ liệu trên giọng nói, để nó có thể chạy hoàn toàn trên mạng dữ liệu. Một số ví dụ là emailto mà một tập tin có thể được đính kèm, do đó thay thế một thông điệp hoặc fax các giải điều chế của một tin nhắn fax vì nó khuyến ters các mạng công cộng để nó có thể được gửi như là dữ liệu thay thế.
Voice networking trends and needs The explosion of Internet traffic and the rapid increase in the number of Internet users has not gone unnoticed. What impact will these developments have on the evolu- tion of existing telecommunications net- works? The volume of data traffic is increasing at a much faster pace than voice or telephony traffic (Figure 1). In the recent past, the traf- fic volume of telephony was much greater than data, and data networking relied main- ly on either using the telecommunications network based on circuit switching, or using separate networks. At some stage, there is a crossover of traffic, such as when both types of traffic occupy the same amount of capac- ity in the network. Nevertheless, voice still accounts for a greater degree of operator rev- enue. But wireline voice traffic is increasing more slowly today, and there are several rea- sons why. A growing segment of voice net- working is handled in cellular networks. Also, new possibilities exist for moving everything that is data-on-voice, so that it can be run entirely over data networks. Some examples are e-mail—to which a file can be attached, thus replacing a fax message—or the demodulation of a fax message as it en- ters the public network so that it can be sent as data instead. At some point, the volume of data traffic will become so much greater than voice traf- fic that it may prove more prudent to run voice on a packetized data communications medium instead of data-on-voice or over separate networks. Some sources predict that this changeover will occur sometime in the next few years. On the terminal or user side, the trend is also toward using packetized voice. This is already true of mobile phones, telephony software clients running on personal com- puters and local area network private branch exchanges (LAN PBX or virtual PBX). The prospect of providing voice transport on bandwidths less than the common 64 kbit/s at lower transmission costs is en- ticing to new operators in deregulated en- vironments, as well as to established opera- tors as a way of staying competitive. As tech- nologies become available and viable for voice coding, efficient software implemen- tations that run on standard personal com- puters or on digital signal processors (DSP)—whose capacity is constantly in- creasing—will be widely used. Yet another trend is the way in which telecom networks and services are evolving from a vertical to a horizontal orientation (Figure 2). The data and voice networking trends de- scribed above are in line with the prospect of a packet-based connectivity network. Thanks to its flexibility and quality-of- service (QoS) guarantees, asynchronous transfer mode (ATM) has proven itself ca- pable of delivering cost-effective switching and transport for a connectivity network. ATM has also been selected for providing switching and transport in the third- generation mobile access networks, which strengthens the case for ATM as a part of the connectivity network. As things stand, voice seems to be mov- ing from being circuit-switched to being packet-switched, which heralds a new op- tion for voice networking. Thus, established operators will have to consider a change of course from circuit-switched to packet- switched networks; but the transition will have to be run smoothly, without a negative impact on services in terms of richness of feature, quality of service, or reliability. Moreover, by using ATM in their networks, new operators will be able to provide voice and telephony in a common network together with data, video and Internet services. Operator and end- user benefits Moving voice networking to ATM benefits the network operator; happily enough, it can also benefit the end-user. In essence, the op- erator’s benefits are wholly economical. However, when it will become more eco- nomical to run voice on an ATM network is 40 Ericsson Review No. 1, 1998 Voice and telephony networking over ATM Jan Höller Voice and telephony still represent the largest volume in today's telecom- munications networks, both in terms of traffic and generated revenue. How- ever, as new communications services are introduced—particularly services generated by applications on the Internet—the telecommunications net- work must evolve to meet the increasing demand. A connectivity network, in common use, that can handle the emerging multitude of applications and services as well as existing services, such as telephony, can reduce opera- tional costs. ATM is a technology that provides the flexibility and strict quality of service required by a network of this type. The author describes how ATM may be used for introducing voice and telepho- ny into a truly multiservice network without reducing the range or capacity of existing services. Thus, operators can enjoy the full flexibility and capabilities of voice and telephony over ATM, which offers seamless interoperability and management, alongside existing telephony services and networks. Box A Abbreviations AAL ATM adaptation layer ADPCM Adaptive differential PCM AM Application modularity ATM Asynchronous transfer mode AVS AXE-served voice services B-ISDN Broadband ISDN CATV Community antenna TV CBR Constant bit rate CE Circuit emulation CS-ACELP Conjugate-structure algebraic- code-excited linear prediction DCME Digital circuit multiplexing equipment DSP Digital signal processor DSS1 Digital signaling system 1 IN Intelligent network ISDN Integrated services digital net- work ISUP ISDN signaling user part ITU International Telecommunica- tion Union IWF Interworking function LAN Local area network NNI Node network interface PBX Private branch exchange PCM Pulse code modulation PRA Primary rate access PSTN Public switched telephone net- work QoS Quality of service RMP Resource module platform SAM Service access multiplexor SDH Synchronous digital hierarchy STM Synchronous transfer mode SVC Switched virtual connection TDM Time division multiplexing UNI User network interface VAD Voice activity detection VBR Variable bit rate VCC Virtual channel connection VPN Virtual private network VPNA Virtual private networking over ATM VTNA Voice transit networking over ATM VTOA Voice and telephony over ATM Ericsson Review No. 1, 1998 41 dependent on individual operator circum- stances. The benefits to end-users are basically two-fold. They can be offered a flexible tar- iff based on the desired quality of service. Thus, a lower voice quality can be chosen as a cheap “tourist class” service. On the other hand, if users so request, a higher audio qual- ity can be provided for certain applications such as conference calls. Furthermore, end- users can opt for integrated access to all ser- vices, making life easier. Equipment and operations cost savings Thanks to the multiservice capabilities of ATM, it is possible to provide a common network for all services. This means that the cost of node equipment is reduced, as it can be shared by all applications. ATM allows common access to residential users as well as to business users, for whom voice is inte- grated with other applications. The edge and core switching equipment is also shared among the different applications. With a smaller amount and variety of equipment in the network, the cost of oper- ation and maintenance is lower. Compared with a synchronous transfer mode (STM) voice network, an ATM-based network, with its higher switching speed, requires less node-interface hardware. Transmission cost savings The obvious benefit that comes to mind when we talk of voice over ATM is its po- tential for lowering transmission costs. Sig- nificant savings can be made on access to ser- vices and other links where transmission is expensive; for example, on international links. Because ATM permits the transmis- sion capacity used in the network backbone to be reduced, operators who own their own transmission networks can sell spare capac- ity as leased lines to third parties. Support for ways of saving on transmis- sion cost is being explored and developed in a number of standardization activities (Box B). If ATM’s capability of supporting on-demand bandwidth or resources is put to use, then we need only reserve as much band- width as is needed at any given instance. By contrast, in a synchronous digital hierarchy Volume Crossover Change- over Time Data Voice Access, transport & switching networks Access, transport & switching networks Services Today's solution Migratory solution Future solution PSTN/ISDN Data (FR etc) Mobile CATV Connectivity networks Service nodes Service networks CATV Mobile Data PSTN/ISDN Figure 1 The volume of data traffic is increasing more rapidly than the volume of voice traffic. At some point in time, the data traffic volume will completely dominate. Figure 2 Networks are evolving away from a vertical orientation toward a horizontal orientation. In the future, several capabilities previously dedicated to specific networks will be com- mon. Specifically, the trend is toward sepa- rated service networks and connectivity net- works. Box B Standards for voice and telephony over ATM In the past two years, standardization bodies have taken an increasing interest in voice and telephony over ATM (VTOA). In a very short time, the ITU-T drafted and approved the new speci- fication, AAL type 2, which was developed with voice over ATM as the target application. Although it is not a standardization body, the ATM Forum has great influence on standard- ization work and is developing a number of implementation agreements for VTOA. This work is currently being carried out by the VTOA working group of the ATM Forum. ITU-T I.363.1 B-ISDN ATM adaptation layer (AAL) specifica- tion types 1 and 2, 1996 I.363.2 B-ISDN ATM adaptation layer specification type 2, 1997 I.Trunk AAL2 SSCS for Trunking, Draft ATM Forum af-vtoa-0078.000 Circuit emulation service interoperability spec- ification v2.0, 1997 af-vtoa-0085.000 Dynamic bandwidth circuit emulation service, 64 kbit/s trunking, 1997 af-vtoa-0089.000 ATM trunking using AAL1 for narrowband ser- vices 1.0, 1997 btd-vtoa-lltaal2-00.02 ATM trunking using AAL2 for narrowband ser- vices, Draft (SDH) network, capacity is reserved on a (semi-) provisioned basis dimensioned to cater for the busy hour. ATM-switched vir- tual connections (SVC), on the other hand, are used to transport voice traffic as demand requires. Bandwidth not used by voice may then be used by other applications (Figure 3). In addition, the introduction of alterna- tive schemes for voice coding with com- pression and silence removal produces vari- able bit-rate (VBR) voice traffic for which the new AAL type 2 (Box C) was developed. This results in still more substantial band- width savings, since compression is used and little or no bandwidth is consumed during silent periods. By its nature, VBR also fa- cilitates gains in dynamic multiplexing. Service-related benefits By preparing the way for a specific migra- tion path, the voice network can evolve smoothly toward an ATM-based network— a smooth migration is much preferred to a drastic replacement. By building on exist- ing services in the public switched tele- phone network (PSTN) and integrated ser- vices digital networks (ISDN), but replac- ing STM transport with ATM, we can en- sure total and seamless interoperability with the existing networks. In this way, parts of the network may be moved to ATM, while other parts remain in STM. The consolidation of PSTN, ISDN and in- telligent network (IN) services provides ser- vice transparency; that is, the service offer- ing is independent of transport technology. It will not only be possible to offer seamless service interoperability, but seamless service management as well. Customers and ser- vices can be centrally managed in the same way regardless of whether they are connect- ed to an STM or ATM network. Existing narrowband services may be consolidated by reusing the network equipment and soft- ware in which investments have already been made. This is described in the two fol- lowing product applications, which were designed to provide full support of voice and telephony services in an ATM network. 42 Ericsson Review No. 1, 1998 Time in seconds/minutes Capacity Capacity used for other applications Bandwidth release Bandwidth allocation Allocated bandwidth Actual telephony load Maximum available capacity Capacity used for voice Box C Circuit-switched vs packetized voice over ATM Preferably, circuit-switched 64 kbit/s pulse-code modulated voice is transport- ed over ATM using ATM adaptation layer type 1 (AAL1). Besides being used for circuit-emulation services, AAL1 supports the transport of constant-bit-rate (CBR) channels such as voice. With the introduction of compressed voice, the bit rate is lower, maintaining typical values of, say, 8 kbit/s for CS- ACELP. The new voice-coding schemes can also be combined with voice activity detection (VAD), which exploits the fact that—on the average—60% of a normal conversation is silence. During these silent periods, the bandwidth used for transmission may be significantly low- ered. This effectively produces a variable- bit-rate (VBR) source out of the speech traffic. With its characteristic CBR at fixed bandwidths, AAL1 is not suitable for this type of traffic. However, a new AAL type 2 has been developed to provide bandwidth-efficient transport of low-bit- rate, real-time services such as VBR voice. Moreover, AAL2 is capable of providing dynamic multiplexing gain as well as reduced delay characteristics. With AAL2, voice and sources of different nominal bit rates can be efficiently multiplexed into the same ATM cell stream while main- taining the strict quality-of-service require- ments for voice services (Figure C1). Thus, AAL2 provides an efficient means of trans- porting packetized voice on ATM. Figure 3 In an ATM network, bandwidth used for voice can be allocated on demand. Momentarily unused bandwidth can be used for other applications. Thus it is possible to maximize the use of resources. Voice Ch 155 Voice Ch 1 AAL2 Common part Voice Ch 7 Voice Ch 1 AAL2 user ATM Silence Ch 3 Silence Ch 77 Data ATM cell ATM cell Packet header Cell header Figure C1 AAL2 allows the multiplexing of voice chan- nels—as well as data—with different charac- teristics into the same ATM cell stream. Ericsson Review No. 1, 1998 43 Virtual private networking over ATM The main goal of providing virtual private networking over ATM (VPNA) is to enable the operator to deliver voice and telephony virtual-private-network services in a cost- effective way, primarily to business cus- tomers. Customers are connected to a multi- service network that uses ATM as the com- mon connectivity layer, as described earlier. Existing PSTN/ISDN and intelligent- network services—the latter being of pri- mary interest to VPNA—provided by AXE are consolidated and deployed on a switched ATM network using the AXD 301. Voice is transported entirely in ATM, end-to-end. The typical broadband network into which VPNA has been built is depicted in Figure 4. The network is deployed to sup- port the communication needs of any size business. The services offered include exist- ing services, such as telephony and data communications, as well as new video and multimedia services. VPNA uses the service access multiplex- or (SAM) as the integrated access equipment for providing all services. The service inter- faces supported by the SAM are primary rate access (PRA) to PBXs, circuit emulation (CE), and a range of data interfaces, such as frame relay and native-ATM—for example, for connecting to routers. The SAMs can be located on customer premises or in a cam- pus environment, thus serving one or more customers. The SAM is connected to a switched ATM backbone network via a standardized ATM user-network interface (UNI). The AXD 301 shown in the figure could be an integral part of the ATM network or it could be connected to the network by a user- network interface (UNI). The AXD 301 supports the telephony application with AXE. The addition of the VPNA features does not require any new functionality in the ATM network other than support for switched virtual connections according to standards. As mentioned above, the virtual private network and intelligent network services, which have successfully been implemented in AXE, are used for voice calls that are switched end-to-end in the ATM network. This is facilitated by separating call control from the bearer services by introducing two new resource types into the resource mod- ule platform (RMP) of AXE (Figure 5): • the switch view—which represents the ATM connectivity that is used for voice and controlled by the AXD 301; • the access view—which handles remote- ly located primary-rate accesses that con- nect PBXs to the service access multi- plexers. The switch view resource type is a virtual switch in the AXD 301 that emulates a switching fabric to be controlled by AXE. The AXE-served voice services (AVS)— which are an AXD 301 software applica- tion—are capable of establishing, control- ling and releasing ATM connections in the network as the telephony call control in AXE requires. To this end, the application relies on the general ATM services provid- ed by the AXD base system. For AXE to provide the full set of ser- vices, such as routing and billing, current narrowband signaling procedures must be retained. Thus, call-control signaling, which uses protocols, such as digital sig- naling system 1 (DSS1) and ISDN user part (ISUP), is transparently transported through the ATM network and terminat- Centrex SAM PBX Customer premises SAM RSS PBX Switched ATM network PSTN/ISDN/PLMN IN Internet AXD301 AXE AXD301 AXE VPN SAM PBX Campus PBX FR, DXI, ATM SS7 PSTN/ISDN SS7 Figure 4 An ATM-based multiservice network that pro- vides voice and telephony services to busi- ness customers. Virtual private networking over ATM consolidates the services of AXE, but switches voice end-to-end in ATM. XSS Resource control AVS AXD base system AXE AXD 301 AM AM AM RMP Figure 5 A separation of call control from the bearer services is facilitated by new resources in the AXE RMP. The AXD 301 emulates the switch- ing fabric to the RMP. AM Application module AVS AXE served voice services RMP Resource module platform XSS Existing source system ed in AXE software (Figure 6). The resource-control protocols required for the access and switch views are terminated in the access equipment and the AXD 301 application, respectively. In this way, AXE and the AXD 301 work much like a tele- phony server. Standard ATM signaling ca- pabilities are used to establish and release connectivity. Maintaining ATM end-to-end means only using one STM-ATM transition for voice, which guarantees toll-voice quality, especially when voice compression is used. Moreover, in the same way that call con- trol is completely separated from the bear- er services, configuration and maintenance of the ATM network are separated from the administration of the telephony services, such as route planning and billing func- tions. In addition to separating call control from bearer services, the AXD 301 provides the VPNA gateway to other networks, public and private, such as to the PSTN, ISDN and cellular networks. Voice transit networking over ATM The product application known as voice transit networking over ATM (VTNA) al- lows large volumes of transit telephony traf- fic to be transported over an ATM network. As mentioned above, VTNA can serve as a path over which parts of the PSTN/ISDN can migrate—using ATM as the common connectivity layer—to become a multi- service network. A part of the migration strategy might be to introduce ATM for handling traffic expansion, or to provide an alternative redundant transit plane. ATM may also be used by new operators who will be providing telephony services in a new en- vironment. In the existing STM-based networks, trunk interfaces are dedicated to specific routes, and trunking transport capacity is provisioned according to busy-hour traffic needs. With VTNA, it is possible for node and network resources to be used more effi- ciently. Because VTNA uses on-demand ATM virtual-channel connections for trans- porting groups of telephony trunks of op- tional sizes, the ATM network bandwidth is shared between the different routes, as are other applications such as data communica- tions. Using ATM also means that the AXD 301 ATM interfaces are shared on- demand between the different routes and ap- plications as the traffic pattern requires (Figure 7). State-of-the-art voice compression is pos- sible on links where transmission costs need to be reduced; for example, in international links where combined voice and data digital-circuit-multiplexing equipment (DCME) capabilities are required. The VTNA application is based on the principle of network interworking (as de- fined in ITU-T Recommendation I.580), where telephony services are provided transparently by AXE using standard ISUP (Figure 8). By using network interworking in this first step, we ensure that service transparency can be achieved—with max- imized reuse of existing applications. The 44 Ericsson Review No. 1, 1998 XSS Resource control (switch view) AVS AXD base system AXE AXD 301 AM AM RMP Call control (e.g. DSS1) Call control PSTN/ISDN/mobile network Switched ATM network IWF IWF PBX PBX Voice path Voice path SAMSAM (e.g. ISUP) Resource control (access view) AXE Defined routes AXE AXD 301 LE3 LE3 LE2 LE2 LE1 LE1 LE3 LE3 Defined routes AXD 301 AXE Defined routes AXD 301 LE1 LE1 LE3 LE2 Exchange 1 Exchange 2 Exchange 3 LE 12 LE 13 LE 23 Route ATM network Allocated trunk group VCCs Load Figure 6 AXE software controls the call services and access and switch-view resources. The voice path, however, can be switched end-to-end in ATM. As depicted in this figure, the AXD 301 and AXE operate as a gateway to the public network. Figure 7 The amount of trunking capacity is allocated depending on traffic load. The ATM network bandwidth is shared between the routes as well as between different applications. Ericsson Review No. 1, 1998 45 ISUP signaling may be embedded in the ATM virtual-channel connections for transport to the far-end AXD 301 and AXE or a separate standard SS7 network may be used. The AXD 301 provides the dynam- ic trunking capabilities as well as the nec- essary interworking with ATM. In this context, AXE is a full-fledged standard AXE. A new type of device, which represents the dynamic ATM trunk group and controls the resources of the AXD 301, has been in- troduced into AXE. When AXE requires more trunk-group capacity, this is commu- nicated to the AXD 301 via a duplicated in- terprocessor bus between the two systems. The AXD 301 then uses standard ATM sig- naling procedures, over a user-network in- terface or node-network interface (NNI), to establish the necessary ATM virtual chan- nel connections through the ATM network to the appropriate destination. By mapping trunk groups—that is, several voice chan- nels—on individual ATM virtual-channel connections, voice delay due to ATM cell packetization can be kept at a minimum, and the signaling load presented to the ATM network can be kept low. Future developments Thanks to the flexibility of ATM, future voice and telephony-like services will be able to support sub-rate voice, or any voice rate at all. The newly standardized AAL type 2 protocol is perfectly suited to achieve this objective. Although the development of AAL2, in which Ericsson was instrumen- tal, was primarily meant to facilitate the use of mobile applications, AAL2 may also be used in wireline networks where VBR voice transport is one of the considerations. The use of compressed voice with adaptive dif- ferential pulse code modulation (ADPCM) and conjugate-structure algebraic-code- excited linear prediction (CS-ACELP) is of particular interest, in that it can provide voice transport down to 8 kbit/s with voice of near-toll quality. Because the product applications pre- sented here support the separation of call control from the ATM bearer services, switched voice over ATM is feasible using any underlying physical infrastructure. One area of interest is in a community antenna TV (CATV) network, where ATM is being supported. Conclusion As the amount of data traffic exceeds the vol- ume of voice traffic, it may prove more vi- able to run voice on data than as separate networks or data-on-voice. There is a grow- ing trend toward transporting voice in pack- etized format. ATM technology is very well suited for voice traffic as it provides on-demand, flexible-bandwidth connections on a large scale. This, in combination with the guar- anteed quality of service that ATM brings to real-time services, makes ATM an ideal choice for circuit as well as packetized voice transport. Building a multiservice network with ATM switching for voice, video and data services provides the operator with a num- ber of cost advantages in the service pro- duction chain. By combining the versatility and flexi- bility of the AXE system with the ATM switching capabilities of the AXD 301 sys- tem, it is possible to consolidate existing PSTN, ISDN and IN services, while at the same time providing voice switching over ATM. This technique is utilized in both the virtual-private networking over ATM and voice-transit networking over ATM prod- uct applications, which support the smooth and seamless migration to a multiservice network of existing networks as well as in- teroperability between new ATM-based networks and existing networks. XSS Resource control AVS AXD base system AXE AXD 301 RMP UNI/NNI Bus STM ATM ISUP signaling ATM network Figure 8 Separate signaling is used for call control and for establishing connectivity. Thus, exist- ing signaling procedures can be retained. References 1 ITU-T I.580, General arrangements for interworking between B-ISDN and 64 kbit/s-based ISDN . networks and connectivity net- works. Box B Standards for voice and telephony over ATM In the past two years, standardization bodies have taken an increasing interest in voice and telephony over ATM. providing voice switching over ATM. This technique is utilized in both the virtual-private networking over ATM and voice- transit networking over ATM prod- uct applications, which support the smooth and. will become more eco- nomical to run voice on an ATM network is 40 Ericsson Review No. 1, 1998 Voice and telephony networking over ATM Jan Höller Voice and telephony still represent the largest